Fiber optic hydrophones are well known in the art for measuring seismic and acoustic disturbances. Generally hydrophones are towed behind a ship to measure these disturbances. However, with the increasing development of subsea or land-based oil/well systems, a hydrophone that could be deployed down a well at extreme depths and that could withstand the extremely corrosive downhole environment would provide significant benefits. Such a hydrophone would improve the ability to explore the land surrounding a well site by seismology or to detect other acoustics downhole that could inform the well operator about various aspect of the well's production.
While hydrostatic pressure has a measurable effect on a hydrophone, especially when the hydrophone is deployed at extreme depths, small dynamic pressures, such as propagating acoustic sound waves, have a relatively small effect and therefore are more difficult to measure. When a measurement is to be made at depths where the hydrostatic pressure is great (e.g., thousands of feet down the well), the hydrostatic pressure can overwhelm the acoustic waves by many orders of magnitude.
In an attempt to resolve relatively small dynamic pressures, fiber optic hydrophones generally have two fiber optic “arms”—a sensing arm and a reference arm. Both the sensing arm and the reference arm generally constitute optical fibers coiled around corresponding cylindrical mandrels—an outer compliant mandrel for the sensing arm and an inner rigid mandrel for the reference arm. The compliant mandrel is typically thin walled so that its radius changes easily in response to the acoustic pressures being measured. A cavity is formed between the two mandrels. A gas (e.g., air) or liquid typically fills this cavity. The rigid mandrel may be relatively thick walled, or alternatively thin walled and exposed to the ambient pressure so that its radius would not change. One such hydrophone is disclosed in U.S. Pat. No. 5,394,377 entitled, “Polarization Insensitive Hydrophone,” and is incorporated herein by reference in its entirety. While compliant mandrels are very sensitive, they are subject to damage and collapse when subjected to extremely high hydrostatic pressures, particularly if they are gas-backed. The production of such gas-backed designs is also costly, largely due to the need to seal the air cavity existing between the sensing and reference mandrels. Furthermore, the reference fiber must enter and exit this air cavity without disrupting the seal. Leaking and fiber breakage at this seal commonly can occur during the assembly process.
An alternative design that attempts to alleviate the problems with gas-backed designs comprises a solid core wrapped with a reference coil of optical fiber. A compliant material is formed around the reference coil such that a cavity is eliminated. Then a sensing coil of optical fiber is wound around the compliant material. Such a design is disclosed in U.S. Pat. No. 5,625,724 entitled, “Fiber Optic Hydrophone Having Rigid Mandrel,” which is incorporated herein by reference in its entirety. While this solid design withstands high pressures when deployed at extreme depths, the design lacks in sensitivity to detect acoustic pressure waves and requires two windings of optical fibers. Other fiber optic hydrophone designs can be found in U.S. Pat. Nos. 5,625,724; 5,317,544; 5,668,779; 5,363,342; 5,394,377, which are also incorporated herein by reference.
The art would benefit from a hydrophone sensitive enough to measure relatively small dynamic pressures while being able to withstand deployment in environments having large hydrostatic pressures. It would be further beneficial for such a hydrophone to contain a single measurement coil, without the need for a reference coil.